Conventional circuit boards having conductive wiring on a substrate have been produced by coating a photoresist on a substrate with a metal foil attached, exposing a desired circuit pattern to the photoresist, and forming a pattern by chemical etching. By this conventional method for producing circuit boards, it is possible to produce high-performance conductive substrates. However, the conventional circuit board production method is disadvantageous in that it has many steps and is complex, while also requiring a photoresist material.
On the other hand, there is increasing interest, in methods of printing of patterns directly on substrates with coating materials comprising dispersed metals or metal oxides. Such methods of printing patterns directly onto substrates do not require use of a photoresist material, and are highly productive methods.
In PTL 1 referenced below, there is proposed a cuprous oxide dispersion containing cuprous oxide microparticles with a mean secondary particle size of 80 nm or smaller, and a polyhydric alcohol having 10 or less carbon atoms. According to PTL 1, the cuprous oxide dispersion has high stability and allows formation of a copper thin-film coating onto a substrate and sintering. Specifically, the cuprous oxide dispersion, having cuprous oxide microparticles dispersed in a diethylene glycol dispersing medium together with polyethylene glycol, undergoes no loss of dispersibility even when allowed to stand overnight. The dispersion is coated onto a glass plate to form a copper thin-film with a thickness of 2.5 μm and a volume resistivity of 8×10−5 Ωcm (see PTL 1, Examples 3 and 6).
In PTL 2 referenced below, there is proposed a metal oxide dispersion including metal oxide microparticles with a mean primary particle size of 20 to 100 nm, a dispersing medium and a dispersing agent. According to PTL 2, the dispersion has high dispersibility at high concentration and low viscosity, and is therefore suitable for ink-jet uses as well.
However, a technique for efficient production of even higher performance conductive substrates has not yet been established. Consequently, no technique has yet been established for efficient production of transistors by direct printing of patterns of electrodes, semiconductors and insulating films onto substrates. In order to form transistor electrode patterns, it is desirable to develop a printing technique that allows formation of more intricately patterned electroconductive films, and a copper and/or copper oxide dispersion that is applicable in the printing technique. In particular, reverse printing methods are suitable for formation of intricate patterns, but no copper and/or copper oxide dispersion has yet been developed that has the dispersibility and coatability necessary for a reverse printing method.